In recent years, automotive vehicles mounted with an auxiliary power supply besides a main battery have been developed for fuel economy improvement and consideration to environments. For example, the amount of power generation by an alternator can be reduced and fuel economy can be improved by accumulating regenerative energy during braking in the auxiliary power supply and using the accumulated energy for the supply of power to electric components during travel. Further, the instantaneous disconnection of the electric components due to an instantaneous voltage drop of the main battery and the deterioration of the main battery can be prevented by supplying power to a starter from the auxiliary power supply when an engine is restarted after an idling stop.
A switching board having a function of switching a power supply mode in correspondence with an operating state of a vehicle such as traveling and an idling stop is disposed between the main battery and the auxiliary power supply in some cases.
Conventionally, boards provided with a mechanical relay compatible with a large current have been used as switching boards. However, it has been proposed to replace mechanical relays with semiconductor switching elements for the purpose of miniaturization, longer service life and noise reduction. It is assumed to use power semiconductors such as MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) as the semiconductor switching elements (see patent literature 1).
Generally, in a switch using a MOSFET, a parasitic diode by a p-n junction is generated between a source and a drain. Accordingly, even if a gate is turned off, a current flows from a source side to a drain side through this parasitic diode, wherefore the flow of the current from the source side to the drain side cannot be completely shut off. Thus, to enable a current to be shut off in both directions, two MOSFETs may be connected in series to reverse an orientation of the parasitic diode.
However, if it is attempted to design such a complicated circuit, the structure of the switching board tends to be complicated.
The present invention was completed based on the above situation and aims to provide a simply configured switching board to be arranged between a main power supply and an auxiliary power supply in a vehicle provided with the main power supply and the auxiliary power supply.